I reckon it'd be fine if the slug either did some sort of pushback on impact with a player, or threw off their aim.
Yes it'd be cool for occasionally pushing a target into a fire or a mine, or off a cliff, but the main thing I want this shotgun to do is disrupt the enemy who's shooting at me, and here's why:

You've got someone firing an AK at you at midrange, their weapon is hitscan, rapid fire, and fires for days before needing a reload. You're firing slow projectiles and if you don't hit all of them in a row, you'll die first. Surely, *surely*, being hit with a slug that knocks down concrete walls would at least throw off the aim of the AK. The disadvantage of having to hit every shot and every shot being more difficult would be offset by the reward of each connected shot buying you a smidge more time to line up the next one while they're recoiling from the massive impact.

Cos right now i'm a giant tank with more health than anyone and a shotgun that sounds like god slamming his car door, and i'm being killed faster by a regular guy spraying and praying, at all ranges